Rig deformation problem
I added a subsurf after the armature modifier, turned on the "preserve volume" option, turned on the "edit mode visibility" buttons and tweaked the weight values: in the central joint edge loop 0.5/0.5, then 0.75/0.25, then 0.95/0.05.

In the arms there is not enough geometry to get smooth bends, you'll have to add some edge loops.